This invention pertains to a color cathode ray tube (CCRT) having a plural-beam in-line electron gun assembly, and more particularly to means for retaining magnetic shunts in the terminal convergence electrode of said gun assembly.
Cathode ray tubes employed in color television and associated display applications commonly utilize an in-line assembly of three electron guns positioned in the neck region of the CRT envelope, from which three separate electron beams emanate in a subtantially common plane. During tube operation, the respective beams are directed along related paths to converge at the plane of a multi-opening color selection electrode or "aperture mask" positioned adjacent to a patterned cathodoluminescent screen. Upon passing through the respective openings or apertures, the converged beams diverge lightly to impinge upon discrete color-emitting areas of the patterned screen.
An associated deflection yoke surrounding the neck in the region of the gun creates magnetic fields which cause the individual beams to scan the screen, thereby producing three different color-raster patterns which conjunctively comprise the screen display.
Technical advancement in in-line CRT's has resulted in simplified dynamic convergence circuitry and a self-converging yoke which automatically corrects for "pin cushion" distortion of the rasters. The resulting raster patterns nevertheless sometimes evidence coma error. That is, their sizes may differ slightly due to the interaction of the yoke fields on the respective electron beams.
To effect coma correction, the prior art has taught the usage of magnetic "shunts" positioned within the gun assembly relative to the plane of deflection to influence or shunt the magnetic fields for particular in-line beams. One successful method of achieving coma correction is to surround the two side-related in-line beams with two disc-like apertured magnetic shielding members at the terminal element or convergence cup of the electron gun assembly. Being so oriented, the shielding members weaken the effect of the yoke-induced horizontal and vertical magnetic deflection fields on the side electron beams.
The shielding members are conventionally affixed to the bottom surface of the convergence electrode by a plurality of spot welds, a procedure which has sometimes produced deleterious effects. Specifically, it has been found that the pressure and heat of welding can sometimes produce localized distortions of the crystal structure of the magnetic shunt material, thereby altering its shunting capabilities. Also, any bending or working of the shunt during affixation could also change its desired shunting characteristics.